This invention relates to balancer shaft assemblies for internal combustion engines.
Balancer shafts are provided in reciprocating engines to counteract vibration associated with large rotating masses. The shafts are driven by the crankshaft, such that the vibrations generated by eccentric weights on the balancer shafts exactly oppose the vibrations caused by the reciprocating pistons. The overall effect is to reduce engine noise and wear on engine mounts and other drivetrain components.
It is usually accepted that the shafts are located such that their axes lie in a plane containing the axis of the crankshaft, but due to engine layout and vehicle size, such a configuration may not always be ideal. EP-A-0.789.165 discloses a lubricating structure for rotary balancer shafts wherein a housing comprises two pieces which vertically sandwich the balancer shafts. The whole structure is then bolted to the engine block below the plane of the crankshaft.
The balancer shafts need to be lubricated in the same way as any other rotating engine component. Conventionally, this has been accomplished by providing drilled oil galleries within the framework or block supporting the balancer shafts. Oil flows from the galleries through exit holes in the pillar blocks to form a layer of lubricating oil between the pillar blocks and the journals on the balancer shaft. This is similar to crankshaft lubrication, and in turn is as expensive and time consuming to fabricate.
The present invention seeks to provide a more cost effective manner of supplying oil to lubricate the balancer shaft journals.
According to the present invention there is provided a balancer shaft assembly for an engine, comprising a housing mountable on the engine block, and rotatably supporting at least one balancer shaft connectable for rotation with the engine crankshaft, the housing being formed in two parts and defining pillar blocks for journalling the, or each balancer shaft.
The assembly is characterised in that a groove is formed in the split plane of at least one of the two mating parts of the housing, which groove, when the parts are mated to one another, defines an oil gallery by way of which lubricant is supplied to pillar blocks supporting the or each balancer shaft.
The invention has the advantage of reducing cost of production as the machining process for creating oil pathways inside the balancer shaft is time consuming and expensive. By providing the oil galleries on the mating surface of the housing, they can be cast as an integral part of the housing.
It is further advantageous to construct the housing as an open ladder frame, thereby reducing weight and making construction more simple. This may however affect rigidity, so with this in mind, the bottom half of the two part ladder frame housing may be formed with reinforcing webs to define a closed drip/splash tray. The webs are provided with drainage holes so that oil is not prevented from falling from the engine and balancer shaft above into the sump below. The webs thus substantially increase the rigidity of the ladder frame housing structure without interfering with the return of oil to the sump. This strengthening in turn adds rigidity to the engine block, by bracing it when the balancer assembly is bolted in place.
In a preferred embodiment of the invention, two counter-rotating balancer shafts are provided. In this case, in order to reduce the number of parts, it is desirable to form the two balancer shafts so that they are identical to each other.
To drive the balancer shafts in opposite directions, it is possible to drive a first shaft at one end by means of a cog driven by the engine and to drive the other balancer shaft by means of a cog mounted on the other end of the first shaft and meshing with an identical cog on the second shaft.
It is preferred, however, to drive the same ends of both balancer shafts. This may be achieved by providing a cog on the first shaft that meshes directly with an engine driven cog and a cog on the second shaft that is driven by the same engine driven cog through an idler gear so as to reverse the direction of rotation of the second balancer shaft relative that of the first balancer shaft.
When the housing is constructed as an open ladder frame, it is possible to provide oil galleries that form a continuous closed loop around the periphery of the split plane, so that lubricant can follow along more than one path (clockwise and counter-clockwise) in order to reach any given bearing. It has been found in practice however that this can result in an excessive drop in the oil pressure (because of increased leakage) and it is preferred to form discontinuous galleries in the split plane of the housing so that oil can follow only a single path in order to reach any given bearing.
With the housing constructed as a ladder network, wherein each rung of the ladder defines two pillar blocks, one for each balancer shaft, it would be possible to interrupt the galleries along the rungs between the two pillar blocks. In this case, each of the galleries running along the stiles of the ladder will carry lubricant to the bearings of a respective one of the two balancer shafts but not the other.
In the preferred embodiment of the invention, however, the pillar blocks on each rung are connected to one another by a lubricant passage formed in the rung, but an oil gallery is formed in only one of the two stiles.